A method and a control device of the type mentioned at the outset for operating a gasoline engine are described in DE 10 2004 060 893 B4. Gasoline engines of the above-mentioned type are increasingly used in systems for decentralized energy generation in combination with a generator. These systems can be used for pure power generation or for a combined generation of power-heat-refrigeration. For the power generation, such systems can preferably be used as emergency power, permanent power, or peak load power aggregates and designed for specific applications.
It is discernible from DE 10 2004 060 893 B4 that during the operation of a gasoline engine, primarily when biogas is used as the fuel, considerable fluctuations of the caloric value and the burning behavior of fuel gas may occur. For example, it is possible that a biogas facility with confermentation shows a CH4 content in the fuel gas with fluctuations ranging from 45 to 65%. In most cases the complementary content in the fuel gas is CO2, so that with an increasing ratio of CO2 the combustion speed of the combustion gas mixture reduces. This may lead to a reduction of the effectiveness and an increase in the exhaust temperature, and perhaps also the NOx emissions, when the ignition time for an ignition device of the gasoline engine is not adjusted. In general, it has been recognized that an adjustment of the ignition time is required to consider the variation of the combustion behavior of the fuel gas during operation of a gasoline engine. According to DE 10 2004 060 893 B4, to the extent possible, technically and economically expensive gas sensors shall be avoided for detecting the combustion behavior of fuel gas. Thus, in order to compensate the above-mentioned disadvantageous influences in a gasoline engine, the determination of the composition of the fuel gas used poses a problem.
According to DE 10 2004 060 893 B4 it has been recognized that it is possible to adjust the ignition time depending on an actual value of the exhaust temperature. For this purpose a target value is predetermined for the exhaust temperature and the ignition time is adjusted for the exhaust temperature to reach the target value. According to DE 10 2004 060 893 B4 it has been recognized that by changing the ignition time the burning behavior and/or the CO2 content of the fuel gas can be compensated in a defined fashion. This provides one method of yielding an essentially constant effectiveness and a defined NO emission while omitting a gas sensor, even when the composition of the fuel gas fluctuates widely.
Such a concept, to some extent successful, has limitations, though, when the exhaust temperature increases due to other influences, such as increasing wear and tear of the engine. In this case, an adjustment of the ignition time to an earlier point, performed according to DE 10 2004 060 893 B4 would lead, under disadvantageous framework conditions, to the gasoline engine developing a knocking operation. In other words, with increasing wear and tear of the engine, a regulation of the ignition time exclusively based on the exhaust temperature leads to the engine no longer being operated according to the gas quality and/or no longer being operated according to the burning behavior of the fuel gas. Reliable operation of a gasoline engine, even in such cases. is desirable.
In order to waive vulnerable gas sensors, namely oxygen sensors, EP 0 259 382 B1 also suggests a method for controlling the air ratio of a gasoline engine. The solution suggested herein fails to provide any adjustment of the ignition time but rather a control of the lambda value of the combustion gas mixture within a required lambda range. For this purpose, in EP 0 259 382 B1 the gasoline engine is adjusted to lean operation by guiding the lambda control depending on the pressure of the combustion gas mixture prior to the inlet valves of the engine and depending on the performance. Constant technical engine parameters are conditional for such a control method, such as fill level and effectiveness of the engine. Although these conditions are largely encountered in practical operation, the widely varying burning behavior of the fuel gas mentioned at the outset leads to strong deviations of the NO emission and the effectiveness from the target values when the ignition time is not adjusted.
In this regard, an improved concept is required in which the flow rate of a fuel gas or a combustion gas mixture is adjustable. In particular, while avoiding expensive gas sensors, an ignition time shall be adjustable according to the composition of the fuel gas or the combustion gas mixture. Particularly the disadvantages shall be avoided to the extent possible, which are connected to a temperature control according to DE 10 2004 060 893 B4.